1. Field of the Invention
The present invention relates to a gripping device for preventing a ring member supporting a manhole cover from being lifted unintentionally. It is particularly directed to a plurality of friction plates secured to the ring member and adapted to be tightened to the cylindrical housing of the manhole. The novel gripping device does not hinder a worker from going down the tubular member in the manhole.
2. Prior Art
U.S. Pat. No. 5,078,539 issued to the present applicant describes a raising structure for manhole cover. Such a structure may be lifted unintentionally by vehicles traveling over a manhole cover and more specifically by a grader or a snow plow scraping the ground around the manhole.
In U.S. Pat. No. 3,926,533 describes the structure of a manhole in which the support for the cover is held by a plurality of staples. U.S. Pat. No. 4,475,845 describes an external manhole chimney seal which may be considered as a means for holding the casting supporting the manhole cover. Such a holding means grips the external surface of the casting and a spinning adjusting ring and is mainly concerned with a sealing function.
H. Bowman in U.S. Pat. Nos. 4,963,053 and 4,872,780 describes a ring member for supporting a manhole cover. The ring member includes a turnbuckle arrangement to tighten the ring member in its housing.
The annular support for manhole cover described by A. Prescott in U.S. Pat. No. 4,995,757 makes use of bolt members radially engaged through the ring support to project the tip of the bolts against a surrounding collar. Considering that the collars are always made of molded cast iron or concrete, they are slightly tapered outwardly to satisfy the molding procedure. Whenever there is a slight movement of the cover and the supporting ring, the tip of the bolts loosen up and the conicity of the collars unlock the supporting ring.